Analog to digital converters typically compare a voltage that has been sampled onto a sampling capacitor with a plurality of voltages to see which voltage best represents the sampled voltage. The plurality of voltages are often provided by a digital to analog converter, DAC. Various analog to digital converter technologies exist. In simplistic designs the voltage on the sampling capacitor is provided to one input of a comparator and the output of the digital to analog converter is provided to the other input of a comparator. The comparator output is supplied to a controller which determines which DAC voltage best approximates the input voltage.
Low power devices are of increasing significance due to battery life and heat dissipation concerns. Generally power dissipation can be reduced by lowering supply voltage. Within an analog to digital converter different components may run at different supply voltages and the voltage at the comparator may be relatively low, say around 2 volts or so.
Using low comparator voltages brings its own problems. The transistors formed within the comparator have parasitic components associated with them, and in particular the formation of switches connected to the input transistors gives rise to parasitic diodes at the comparator input, and if the input to the comparator falls outside the power supply range for the comparator then these parasitic diodes can switch on thereby allowing charge to be lost from the sampling capacitor. If this happens then the result of the analog to digital conversion process is unreliable. Also, voltage excursions outside of the supply rails may damage a device.